In molecular spintronics, the spin state of a molecule may be switched on and off by changing the molecular structure. Here, we switch on and off the molecular spin of a double-decker bis(phthalocyaninato)terbium(III) complex (TbPc2) adsorbed on an Au(111) surface by applying an electric current via a scanning tunnelling microscope. The dI/dV curve of the tunnelling current recorded onto a TbPc2 molecule shows a Kondo peak, the origin of which is an unpaired spin of a π-orbital of a phthalocyaninato (Pc) ligand. By applying controlled current pulses, we could rotate the upper Pc ligand in TbPc2, leading to the disappearance and reappearance of the Kondo resonance. The rotation shifts the molecular frontier-orbital energies, quenching the π-electron spin. Reversible switching between two stable ligand orientations by applying a current pulse should make it possible to code information at the single-molecule level.

We present a systematic investigation of molecule-metal interactions for transition-metal phthalocyanines (TMPc, with TM = Fe, Co, Ni, Cu) adsorbed on Ag(100). Scanning tunneling spectroscopy and density functional theory provide insight into the charge transfer and hybridization mechanisms of TMPc as a function of increasing occupancy of the 3d metal states. We show that all four TMPc receive approximately one electron from the substrate. Charge transfer occurs from the substrate to the molecules, inducing a charge reorganization in FePc and CoPc, while adding one electron to ligand π-orbitals in NiPc and CuPc. This has opposite consequences on the molecular magnetic moment: in FePc and CoPc the interaction with the substrate tends to reduce the TM spin, whereas in NiPc and CuPc an additional spin is induced on the aromatic Pc ligand, leaving the TM spin unperturbed. In CuPc, the presence of both TM and ligand spins leads to a triplet ground state arising from intramolecular exchange coupling between d and π electrons. In FePc and CoPc the magnetic moment of C and N atoms is antiparallel to that of the TM. The different character and symmetry of the frontier orbitals in the TMPc series leads to varying degrees of hybridization and correlation effects, ranging from the mixed-valence (FePc, CoPc) to the Kondo regime (NiPc, CuPc). Coherent coupling between Kondo and inelastic excitations induces finite-bias Kondo resonances involving vibrational transitions in both NiPc and CuPc and triplet-singlet transitions in CuPc.

We present a method for including inelastic scattering in a first-principles density-functional computational scheme for molecular electronics. As an application, we study two geometries of four-atom gold wires corresponding to two different values of strain and present results for nonlinear differential conductance vs device bias. Our theory is in quantitative agreement with experimental results and explains the experimentally observed mode selectivity. We also identify the signatures of phonon heating.

We present novel methods implemented within the non-equilibrium Green function code (NEGF) transiesta based on density functional theory (DFT). Our flexible, next-generation DFT-NEGF code handles devices with one or multiple electrodes (Ne ≥ 1) with individual chemical potentials and electronic temperatures. We describe its novel methods for electrostatic gating, contour optimizations, and assertion of charge conservation, as well as the newly implemented algorithms for optimized and scalable matrix inversion, performance-critical pivoting, and hybrid parallellization. Additionally, a generic NEGF "post-processing" code (tbtrans/phtrans) for electron and phonon transport is presented with several novelties such as Hamiltonian interpolations, Ne ≥ 1 electrode capability, bond-currents, generalized interface for user-defined tight-binding transport, transmission projection using eigenstates of a projected Hamiltonian, and fast inversion algorithms for large-scale simulations easily exceeding 10 6 atoms on workstation computers. The new features of both codes are demonstrated and bench-marked for relevant test systems. * The transport of charge, magnetic moments and, in general, any sort of excitation is a fascinating fundamental physical problem that has demanded attention for a long time [1]. Today, the interest is enhanced by the technological needs of an industry increasingly based on devices whose detailed atomistic structure matters [2], but the treatment of transport is still a formidable open task. Spurred by the fast developments of the microelectronic industry, the first attempts to understand electronic transport at the atomic scale where based on scattering theory [3]. The electron transmission between two semi-infinite reservoirs was treated in a time-independent fashion solving the scattering matrix connecting the reservoirs. At this stage, transport was described as one-electron scattering by a static contact region and this granted access to many concepts and to devising new experiments [4][5][6]. However, the problem is fundamentally a non-equilibrium one that requires evolving many-body states [7][8][9][10].Density functional theory (DFT) has been one method to address some aspects of this problem. Conceptually, DFT is a mean-field many-body theory of the ground state. As such, it can in principle give exact results for the linear conductance because the linear response is a property of the ground state [11]. Beyond linear conductance, not even ideal DFT works because of the need to describe excited states and dynamics of the system. Such limitations may be mitigated by using time-dependent DFT [12,13], but going beyond the linear regime is highly nontrivial. A main issue of a DFT description stems from the approximations made to compute the ground state. Indeed, it has been recently shown that cases where strong correlations rein, such as the Coulomb blockade regime, the commonly used exchange-and-correlation functionals fail and new ones have to be used [14].Probably the most significant conceptu...

The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.

We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmeties in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states -namely those belonging to the most transmitting eigenchannels -need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.Electronic transport through atomic-size junctions is of immense scientific and technological interest. The importance of inelastic effects in electronic currents have been revealed in several ground-breaking experiments leading to the detection and identification of single molecules [1], chemical reactions [2,3], the detection of vibrations in atomic wires [4], the detection of inelastic effects by fluorescence [5], the modification of electron transport in nanotubes [6], the molecular motion induced by electronic currents [7], and the hydrogen detection in atomic wires [8], just to cite a few examples. Of particular importance due to its spreading use is the case of vibrational spectroscopy where the conductance changes due to phonon emission is measured [9,10,11]. This is often referred to as point contact spectroscopy or inelastic electron tunneling spectroscopy (IETS) [1], However, experiments alone are not able to give direct insight into the fundamental question on how the detailed atomic structure correlate with the electrical transport properties. There is experimental evidence of approximate selection rules (propensity rules [12]) such that only a small number out of the many possible vibrational modes give an inelastic signal. These propensity rules yield clues to the geometric and electronic structure of the junctions. It is therefore of fundamental interest to compare the experimental results with first-principles calculations.Existing calculations of inelastic effects in electron transport have been developed either for particular cases [12,13,14] or for simplified (one-level) models [15,16]. First-principles methods capable of treating both weak and strong coupling to the electrodes has also been developed [17,18,19]. However, the results of such detailed calculations involve many electronic states and vibrational modes. An advanced analysis is therefore needed in order to provide insight into the propensity rules.In this paper we propose a method for analysis of the inelastic transport based on just a few selected electronic scattering states, namely those belonging to the most transmitting eigenchannels at the Fermi energy (ε f ) [20]. These scattering states typically have the largest amplitude inside the junction and thus account for the majority of the electron-phonon (e-ph) scattering. To illustrate our method of analysis and to develop an understanding of the propensity rules we consid...

We have carried out a density functional study of vibrationally inelastic tunneling in the scanning tunneling microscope of acetylene on copper. Our approach is based on a many-body generalization of the Tersoff-Hamann theory. We explain why only the carbon-hydrogen stretch modes are observed in terms of inelastic and elastic contributions to the tunneling conductance. The inelastic tunneling is found to be efficient and highly localized in space without any resonant interaction and to be governed by a vibration-induced change in tunneling amplitude.

The excitation of the spin degrees of freedom of an adsorbed atom by tunneling electrons is computed using a strong coupling theory. The excitation process is shown to be a sudden switch between the initial state determined by the environmental anisotropy to an intermediate state given by the coupling to the tunnelling electron. This explains the observed large inelastic currents. Application is presented for Fe and Mn adsorbates on CuN monolayers on Cu(100). First-principles calculations show the dominance of one collisional channel, leading to a quantitative agreement with the experiment.PACS numbers: 68.37. Ef, The way electrons flow through atomic contacts has important fundamental and technological implications [1]. Electronic transport is a quantal process in which charge, spin and vibrational degrees of freedom are entangled leading to problems of intrinsic fundamental interest. Technologically, the quest for minutarization is pushing the limits of devices to the atomic scale, where the above transport properties will determine the actual device functionalities. An important issue is the appearance of inelastic effects where energy is taken from the electron flow into the different degrees of freedom of the system. Inelasticities lead to new regimes of transport that contain relevant information on the atomic contact and have been thus used to develop single atom and molecule spectroscopies [2,3,4].Inelastic electron tunneling spectroscopy (IETS) where electrons excite vibrations leading to conductance steps at certain voltage thresholds [2] has been extensively studied in the last years [5,6,7,8,9]. The inelastic change in conductance is within a few percent of the elastic conductance, mainly due the smallness of the electron-vibration coupling [10,11]. Recently, Heinrich and co-workers have been able to develop a spin-resolved spectroscopy using an STM [4,12,13,14]. In magnetic IETS [4], the tunneling electron yields energy to the spin of an adsorbed magnetic atom and in this way changes its orientation by overcoming the magnetic anisotropy barrier of the atom on the surface. Magnetic transitions in the meV range could be observed in adsorbates partly decoupled from a metal substrate [4,12,13,14,15]. As in vibrational IETS, the conductance presents a step at the energy threshold however the changes in conductance at inelastic threshold can reach several hundreds percent. This is at odds with previous treaments [13,16,17] where first-order perturbation theory is used.In this letter, we present an all-order theory of the spin transitions IETS and apply it to the cases of Fe and Mn adsorbates on a CuN monolayer on Cu, experimentally studied in Refs. [13,14]. We compute the relative weights of both elastic and inelastic channels, leading to a quantitative account of the inelastic currents in the experimental observations. The theory reveals the nature of the inelastic transitions and explains the extremely large inelastic currents in these magnetic systems.The general idea of our approach is the following. ...